The present invention relates to an optical system wherein a light beam emitted from a light source is converged on an optical information recording medium and thereby optical information is recorded and reproduced, and particularly, to an optical system wherein a change in temperature has less influence.
As a conventional optical system for recording and reproducing for an optical information recording medium which is required to have accuracy at the level for coping with CD (an optical system for recording and reproducing in the invention includes an optical system for recording, that for reproducing and that for both recording and reproducing), an optical system of an infinite conjugation type is disclosed in Japanese Patent Publication Open to Public Inspection No. 76512/1982 (hereinafter referred to as Japanese Patent O.P.I. Publication), and an optical system of a finite conjugation type is disclosed in Japanese Patent O.P.I. Publication No. 56314/1986. Further, Japanese Patent O.P.I. Publication No. 258573/1994 discloses an optical system wherein a coupling lens is used for preventing the occurrence of aberration caused by temperature change in the case of a lens made of resin used.
Recently, however, high density recording on an information recording medium such as an optical disk is further advancing, and this causes a numerical aperture (NA) value in an optical system or of an objective lens to be higher. In addition to this, requirements for performance aspects such as wavefront aberration (spherical aberration) are becoming more severe.
An optical system wherein diverged light emitted from a light source is focused on a recording surface of an optical information medium by an objective lens of a finite conjugation type whose spherical aberration and sine condition are corrected, is well-known. In this case, however, following problems are caused when numerical aperture NA takes a greater value because a refracting power of each surface is greater.
(1) There is a limitation for higher NA.
(2) An amount of spherical aberration generated when an objective lens is moved in the direction of an optical axis for focusing is large.
(3) Occurrence of spherical aberration caused by a change in refractive index of an objective lens is great.
When coping with such higher NA and higher accuracy, wavefront aberration caused by a change in distance between an object and an image resulted from unintentional movement of a disk, or by a change in refractive index resulted from ambient change such as temperature change in the case of a lens made of resin becomes great. Further, requirements in performance which are becoming more severe cause tolerance for an objective lens to be more severe than in the past, and in certain circumstances, there is a possibility that no error is allowed.
When an objective lens is made of resin, in particular, though the required level of conventional accuracy for coping with CD has been satisfied by a method employing a coupling lens disclosed by Japanese Patent O.P.I. Publication No. 258573/1994 in the case of a finite conjugation type, the performance required for coping with the recent high density recording can not be satisfied.
In the case of an infinite conjugation type, a change in wavefront aberration caused by a change of a distance between an object and an image does not exist. However, when numerical aperture NA is enhanced to a level of about NA 0.60, a tolerance for the change in wavefront aberration caused by its temperature change is made to be severe at the capacity required for coping with high density recording.
As an example, when a lens made of resin having focal length F of 3.36 mm and NA of 0.6 is a lens of an infinite conjugation type (parallel light coming from the light source side), a wavefront aberration changes by about 0.043 .lambda. (.lambda.=635 nm) for the temperature change of 30.degree. C. Even such small change actually causes a considerable restriction in the required accuracy for coping with DVD announced recently.
The invention further relates to an optical system for reproducing of an optical information medium, a pickup device for an optical information medium, an objective lens for optical information recording and reproducing used for the optical information pickup device and for the optical system for recording and reproducing of an optical information medium, and to a converging lens for an optical system for recording and reproducing optical information.
FIG. 63 shows an example of a pickup device for optical information composed of a conventional optical system for recording and reproducing for an optical information medium. In the drawing, a light flux emitted from light source 1 such as a semi-conductor laser or the like enters collimator lens 3 through beam splitter 2 to be converted to a parallel light flux which is stopped down by aperture stop 5 to a prescribed light flux, and it enters objective lens 6. This objective lens 6 forms, when parallel light flux enters it, an image of a light spot which hardly has an aberration on information recording surface 8 through transparent substrate 7 having a prescribed thickness.
A light flux modulated by information pit and reflected on information recording surface 8 passes through objective lens 6 and collimator lens 3 to return to beam splitter 2 where the light flux is separated from a path of light emitted from laser light source 1 and enters light detector 9. The light detector 9 is a multi-split PIN photodiode which outputs from its element an electric current that is proportional to the intensity of an incident light flux and sends the electric current to an unillustrated detecting circuit. In the detecting circuit, the light detector controls objective lens 6 with a 2-dimensional actuator composed of a magnetic circuit and a coil based on focus error signals and track error signals, and thereby makes a position of a light spot to be on an information track constantly.
In the pickup device of optical information medium as in the foregoing, large NA (for example, NA 0.6) is used for making a light spot converged by an objective lens small. Therefore, when a thickness of a transparent substrate placed in such a converged light flux is deviated from a prescribed thickness, a serious spherical aberration is caused.
For example, when a substrate thickness is changed for an objective lens optimized under the conditions of NA 0.6, wavelength of 635 nm for a laser beam emitted from a laser light source, a substrate thickness of 0.6 mm and of substrate refractive index of 1.58, an aberration increases by about 0.01 .lambda. rms for a deviation of 0.01 mm in the substrate thickness as shown in FIG. 64. Therefore, when a thickness of a transparent thickness is deviated by .+-.0.07 mm, it causes an aberration of 0.07 .lambda. rms which means that the aberration reaches Marechal criterion (0.07 .lambda. rms) that is a standard with which reading can be conducted normally.
In an example shown in FIG. 63, therefore, when a thickness of transparent substrate 7 is changed from 0.6 mm to 1.2 mm, objective lens 6 suitable for 0.6 mm thickness is changed to objective lens 111 suitable for 1.2 mm thickness and aperture stop 10 is changed, for reproduction.
Further, as another method to cope with a change from 0.6 mm to 1.2 mm in terms of a substrate thickness, it is also considered to provide two pickup devices one of which is for a 0.6 mm-thick substrate and the other is for a 1.2 mm-thick substrate.